A Raman spectroscopic investigation of the mechanism of the reduction in hair with thioglycerol and the accompanying disulphide conformational changes.

OBJECTIVE: The objective of our research was to investigate the reduction mechanism of thioglycerol (TG) on hair keratin fibres.

METHODS: The structure of cross-sections at various depths of virgin white human hair resulting from the permanent waving process with TG was directly analysed at the molecular level using Raman spectroscopy. Also, the penetration of TG for the cross-sectional samples dyed with methylene blue was observed by optical microscopy.

RESULTS: The gauche-gauche-gauche (GGG) and gauche-gauche-trans (GGT) conformations of disulphide (-SS-) groups remarkably decreased, while the trans-gauche-trans (TGT) conformation increased by performing the reduction process with TG. In addition, the disconnected -SS- content at various depths of the hair sample reduced with TG adjusted to pH 9.0 with ammonia solution was clearly increased compared with that of the hair sample reduced with TG adjusted to pH 9.0 with monoethanolamine. In the case of adjusting to pH 9.0 with ammonia solution, the tensile strength of the waved hair treated with TG was strong compared with that of the waved hair treated with thioglycolic acid (TGA), but the waving efficiency of the waved hair treated with TG was nevertheless higher than that of the waved hair treated with TGA.

CONCLUSION: The author concluded that the waved virgin human hair treated with TG adjusted to pH 9.0 with ammonia solution was less damaged as compared with the waved hair treated with TGA, despite its good waving efficiency, as not only were the GGG and GGT conformations disconnected, but they were also changed to the TGT conformation by performing the reduction process with TG.